1. Field of the Invention
The present invention pertains to the art of cooking appliances and, more particularly, to a microwave cooking appliance including a system for sensing the presence of a load in an oven cavity of the cooking appliance.
2. Discussion of the Prior Art
Cooking appliances utilizing a directed microwave energy field to cook a food item have existed for some time. In general, a cooking process is performed operating a microwave emitter, such as a magnetron, to direct standing microwave energy fields into an oven cavity such that the microwave energy fields reflect about the oven cavity and impinge upon the food item. As the microwave energy fields impinge upon the food item, the energy fields are converted into heat through two mechanisms. The first mechanism, ionic heating, results from the liner acceleration of ions, generally in the form of salts, present within the food item. The second mechanism is the molecular excitation of polar molecules, primarily water, present within the food item. Regardless of the particular mechanism, the nature of the standing waves results in localized areas of high and low energy which cause the food to cook unevenly. This is especially true in larger ovens where the size of the cavity requires a more uniform energy distribution in order to properly cook the food. To attain an even or uniform energy distribution, the microwave energy must be introduced into the oven cavity in a manner which creates a constructive standing wave front which will propagate about the oven cavity in a random fashion.
Another area of concern in microwave cooking is microwave energy fields being directed into an empty or substantially empty oven cavity. Without the presence of a load, the microwave energy fields could damage interior portions of the oven cavity. In addition, the microwave energy fields could reflect back into the magnetron causing damage to internal structure of the magnetron. In recognition of this problem, the prior art has proposed several solutions. For example, U.S. Pat. No. 5,550,355 discloses a microwave oven having a load sensing system. The oven includes a control that measures oven cavity temperature. If, based on the oven cavity temperature, the control determines that there is no load in the oven cavity, the oven is shut down. In another arrangement, disclosed in U.S. Pat. No. 3,412,227, a neon tube is mounted within a microwave oven. In the event that no load is present, reflective energy in the oven will become substantial enough to illuminate the neon tube. In turn, the neon tube activates a photocell that, upon sensing light from the neon tube, signals a control to discontinue operation of the oven. While each of the above devices is effective, the overall response time is slow, allowing significant energy to still be directed into an empty oven cavity. Over time, the cumulative effects of running the oven with no load could lead to damage to both the oven cavity and the magnetron.
Based on the above, there still exists a need for an effective load sensing device in a microwave oven. More specifically, there exists a need for a load sensing device in a microwave oven that has a short response time so that, in the event that the microwave oven is operated without a load in the oven cavity, operation of the microwave oven will be immediately terminated.